The Turkish foreign ministry - which has summoned Swiss diplomats in Ankara, demanding legal action - says the protest was organised by the outlawed Kurdistan Workers Party (PKK). The PKK is designated a terrorist organisation by Turkey, the European Union and the US, but not by Switzerland.

Despite the tension, voting began Monday at airports and border gates in Turkey, as well as in Turkish diplomatic missions in Germany, Austria, Belgium, France, Switzerland, and Denmark.

In Germany, voters were divided over the vote.

Image copyrightEPAImage caption
Almost half of the three million eligible voters outside of Turkey live in Germany

Ayfer Inci-Pekoz, who was handing out flyers in support of "no" in Berlin, told the Associated Press news agency : "This referendum scares me. I think if Erdogan wins it will further erode democracy in Turkey."

But Senol Akkaya said people in Germany "almost feel guilty" if they support "yes".

"There's this obsession in Germany now, that if you're for Erdogan, it automatically means you're anti-democratic," he told AP.

Ercan Yasaroglu said the referendum had caused tensions within the Turkish community, and between Germans and Turks - reducing it to a test of loyalty to their adopted homes.

"Democracy means also to tolerate the opinions of your opponents," he said. "We've all done a pretty bad job at this recently. Let's try to focus again on what we all have in common, on our humanity, and on building bridges instead of tearing them down."

Voting will start in other countries in due course, with eligible voters able to cast their ballots at 120 missions in 57 countries.

All voting outside the country will end on 9 April, with the main referendum vote taking place on 16 April in Turkey itself.